It has become common for entities other than power utilities to generate electric power and to provide it to the utility grid, thereby to derive revenue or offset customer electric utility bills. Recapture and utilization of energy (such as heat) which would otherwise be wasted, as a byproduct of some useful function, is becoming commonplace.
In some electric power generating systems, the manner of harnessing the energy that will operate the generator may require auxiliary equipment, such as pumps and fans. An example is a combined heat and power system which recovers waste heat, such as from food processing plants or landfills utilizing an organic Rankin cycle system as disclosed in publication US 2004/0088985A1. In such a system, a compressible fluid has heat added and rejected at constant pressure along with isotropic expansion and compression, as is well known. The process may be organic, in which the working fluid changes states, or there may be a process in which a working fluid does not change states.
For economic efficiency, it is desirable that a low cost generator provide power for all auxiliary equipment, while at the same time presenting power which has shape (without harmonic distortion), power factor and frequency that are all suitable for interface with the utility power grid. Synchronous generators are expensive and require additional controls compared with other, cheaper generators such as induction generators, in which the rotor consists of simple conductive bars, short-circuited at the ends, which is much less expensive than synchronous generators. However, induction generators have an inherently lower power factor than what is acceptable to utility grids.
As a specific example of harmonic distortion, consider an electric power generator 8, shown in FIG. 1, which produces 270 kW, connected to a grid 9 by breakers 10 and to auxiliary variable speed motors 11, 12 which consume 70 kW total. The net power available for supply to the grid 9 is 200 kW. The variable speed motors 11, 12 are powered by insulated gate bipolar transistor (IGBT) switched bridge converters 13, having switching controllers 14, the DC input of which 15 is provided by three-phase diode rectifiers 16, as is illustrated in FIG. 1.
A typical best case harmonic distortion of current at the utility grid 9, due to the auxiliary apparatus 11–16, may be on the order of 32%. This amounts to about 8% distortion of the 200 kW being generated at the utility interface. Typical requirements of a power utility include harmonic distortion of less than 5%. Although harmonic filter traps 18 attached to the power bus 17 may be used, they will be application-specific, requiring tuning in each case—that is, in each application design. Apparatus on the grid 9 may interact with the filter traps 18, so that the filter traps will be absorbing harmonic energy from the grid, thereby stressing the components above the intended rating. The filter traps may result in the power factor to be more lagging. Harmonic filter traps also increase the cost and space requirements of the installation where used.
The use of variable speed motors 11, 12 driven by an induction generator causes the power factor to be very lagging, and thus lower than that typically required by a utility (ranging between 0.85 and 0.95). This may be corrected by large power factor correcting capacitors 20, which increase the cost of the system, consume space, and are possible sources for self-excitation, all of which can be inappropriate in many installations. These capacitors may require additional bulky series inductors for limiting harmonic current to the capacitors.
Another requirement for providing generated power to a utility power grid is referred to as “anti-islanding”, which requires that the power generator be disconnected from the grid whenever the voltage, frequency or power factor become out of certain limits. This may be accommodated by monitoring power factor, since any differences between the voltage or the phase of the generated power and that on the power utility grid will alter the power factor sufficiently to be detectable, and cause tripping of interconnection breakers.